Many pyrrolizidine alkaloids and their N-oxides have antitumor activity against a variety of animal tumor model systems. Hepatotoxicity in animals has discouraged their study as clinical antitumor agents with the exception of indicine N-oxide which is currently undergoing clinical trials and has striking activity against acute refratory leukemia. The objectives of this proposal are to determine the physicochemical properties of pyrrolizidine alkaloids (and N-oxides) associated with antitumor activity, to study the mechanism of action responsible for antitumor activity. The hepatotoxicity of pyrrolizidine alkaloids (and N-oxides) is thought to be due to metabolism of the tertiary bases by liver enzymes to chemically reactive dehydropyrrolizidine metabolites. The hepatotoxicity and antitumor activity of pyrrolizidine alkaloids with similar or identical parent ring structures vary markedly with minor changes in the ester side chain moiety. We have clinic and laboratory evidence that the antitumor We have clinical and laboratory evidence that the antitumor activity of these compounds may not be mediated by the pathway responsible for hepatotoxicity. The physicochemical properties of the ester moiety appear to play important roles in the pharmacological activity of these compounds. We are preparing series of semisynthetic pyrrolizidine alkaloids (and N-oxides) differing in physicochemical properties including lipid solubility, rates of chemical and enzymatic ester hydrolysis, and electronic and steric factors of the ester side chains which may influence the biologic activity of these compounds. Metabolism of selected semisynthetic and and natural occurring pyrrolizidine alkaloids (and N-oxides) will be studied in detail. Cytotoxicity will be determined in the human stem cell colony forming assay for drug sensitivity which we have modified to include a liver drug metabolizing system for assessment of the role of biotransformation in antitumor activity. we will correlate antitumor activity with chemical structure and physicochemical properties to determine characteristics involved in antitumor activity. From these studies, we hope to prepare synthetic pyrrolizidine alkaloids and N-oxides with greater antitumor activity using a new synthetic pathway for the total synthesis of pyrrolizidine alkaloids and their N-oxides.